By-products of many desulfurization redox processes include solid elemental sulfur suspended in a liquid redox solution. In some liquid redox processes, it is desirable and necessary to use a sulfur melter to melt the solid elemental sulfur to produce a high quality, marketable sulfur product. However, metallic ions in the redox solution, such as iron and vanadium, react with hydrosulfide, thiosulfate and bicarbonate ions, collectively called “reactive solutes,” at elevated temperatures, forming metallic polysulfides. These metallic polysulfides are undesirable in the context of producing high quality elemental sulfur. The formation of a high quantity of metallic polysulfides may render the sulfur unusable, and also cause fouling in the melter, requiring subsequent cleaning of the melter tubes.
The rate at which metallic ions react with sulfur is a function of the quantity of metallic ions in the redox solution, the melter temperature, the amount of time the sulfur is in contact with the redox solution at elevated temperatures, and the surface area of the interface between the molten sulfur and the redox solution. If more metallic ions are present in the solution, more polysulfides will be formed. As the melter temperature increases, the reaction activity between sulfur and metallic ions increases, forming more metallic polysulfides. As the contact time between the molten sulfur and the redox solution at high temperatures increases, more metallic polysulfides will be formed. The interface between the molten sulfur and the redox solution provides constant contact of the molten sulfur and the metallic ions. Thus, a smaller interface surface area between the molten sulfur and the redox solution will limit the formation of metallic polysulfides.
A filter/wash/reslurry system may be employed to reduce the metallic ions and reactive solutes entering the sulfur melter. Also, sulfur melters may be operated at the lowest possible temperature above the melting point of the sulfur. Although limiting the metallic ions and reactive solutes entering the melter via filtration and operating the melter at lower temperatures are effective techniques in improving sulfur quality, residence time and interface surface area also play significant roles in the formation of metallic polysulfides and consequently sulfur quality. Even when the melter temperature is maintained at the lowest possible level in conjunction with a filter/wash/reslurry system, sulfur quality will deteriorate when long residence times and large interface areas are employed.
With conventional sulfur separation designs, residence time is one of the least controllable variables affecting sulfur quality. Sulfur separators are generally designed to provide a specific residence time for phase separation corresponding to the maximum sulfur throughput of the unit, and determined largely by the aqueous volumetric flow. As residence time increases beyond the time expected during maximum sulfur production, sulfur separation improves because the sulfur droplets have more time to separate from the redox solution. In addition, the interface level between the redox solution and the molten sulfur is more sharply defined, and thus interface level control is improved. However, as residence increases, the formation of polysulfides increases. Thus, an optimum residence time is generated based on a compromise between these considerations.
Prior known designs control the flow of molten sulfur by maintaining the interface level at a certain vertical level. Examples of these designs are described in U.S. Pat. Nos. 4,730,369 and 5,651,896. These known separators are liquid full vessels and do not have a gas phase. The principle is that the operating pressure of the vessel is maintained at a pressure set point to keep the aqueous phase from vaporizing when operating at or above the melting point of sulfur. However, in actual operation, when the molten sulfur control value (interface level control) or the aqueous solution control value (pressure control) opens, the pressure within the vessel drops and a portion of the aqueous phase vaporizes. This vaporization causes severe operating problems such a molten sulfur carryover out of the top of the vessel and the resultant plugging due to freezing of the molten sulfur in downstream equipment and piping. Our invention now solves this and other problems by including a third fluid phase, namely a gas phase, in the main separator vessel with a separate control system to maintain the pressure of the vessel regardless of the level of the aqueous phase or the molten sulfur phase. These and other advantages will become evident from the following more detailed description of the invention.